That's the girl who made the poster!

'S e sin an nighean a rinn am postair

a | nach 

We use a in the middle of a sentence, and before a verb, to mean who, which, that . It's also known as the relative particle. When we use it join to two independent clauses with a positive verb, we use the independent form of the verb. 

To join a negative clause in this way we use nach (who, which, that not). As with the question càit(e) , nach uses the dependent form of verbs. 

'S e sin an nighean.  Rinn i am postair.  'S e sin an nighean a rinn am postair. 
That's the girl.  She made the poster.  That's the girl who made the poster. 
Seo an duine. Cha d' fhuair e duais. Seo an duine nach d' fhuair duais. 
This is the man. He didn't get a prize. This is the man who didn't get a prize 

Play with the sentences until you are happy with a and nach

'S e seo an nighean

This is the girl who

a

a

'S e seo an duine

This is the man who